Morphological differences in the female anal sphincter complex between endoanal and exoanal ultrasound.

Introduction and hypothesis: This study was aimed at describing the morphological characteristics of the normal female anal sphincter complex (ASC) by high-resolution transperineal ultrasound (TPUS) and endoanal ultrasound (EAUS). Methods: Thirty volunteers, including 19 nulliparous and 11 women who had only given birth by Cesarean section, were recruited. The ASC was shown in a set of five slices by EAUS and 8 slices by TPUS. EAUS slices 1 and 5 matched TPUS slices 1 and 8; EAUS slices 2, 3, and 4 were approximately at the levels of TPUS slices 3, 5, and 7. The anterior and lateral EAS and IAS thicknesses were measured and the two methods were compared. Results: The median age was 29 years. The mean body mass index (BMI) was 20.6 kg/m2. All measured values obtained by TPUS were normally distributed. The EAS thickness on EAUS slice 2 and the IAS thickness on EAUS slices 2 and 3 were not normally distributed. The mean anterior EAS thickness was 2.51 mm, 2.96 mm, and 3.53 mm in slices 3, 5, and 7 on TPUS and was significantly thinner than that in slices 2 (2.62 mm), 3 (3.77 mm), and 4 (4.62 mm) on EAUS. In contrast, the IAS was markedly thicker on TPUS. The ratio of sphincter thickness anteriorly to laterally was similar using the two methods. Conclusions: The morphological characteristics of EAS shown by TPUS and EAUS were similar. EAS was thinner, and IAS was thicker on TPUS. The IAS seems to be distorted on endoanal imaging. [ABSTRACT FROM AUTHOR]